1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to generating power from radio frequency signals.
2. Description of Related Art
A radio frequency identification (RFID) system generally includes a reader, also known as an interrogator, and a remote tag, also known as a transponder. Each tag stores identification data for use in identifying a person, article, parcel or other object. RFID systems may use active tags that include an internal power source, such as a battery, and/or passive tags that do not contain an internal power source, but generate power from radio frequency (RF) signals received from a reader.
In general, to access the identification data stored on an RFID tag, the RFID reader generates a modulated RF interrogation signal designed to evoke a modulated RF response from a tag. The RF response from the tag includes the coded identification data stored in the RFID tag. The RFID reader decodes the coded identification data to identify the person, article, parcel or other object associated with the RFID tag. For passive tags, the RFID reader may also generate an unmodulated, continuous wave (CW) signal from which the passive tag derives its power.
RFID systems typically employ either far-field technology, in which the distance between the reader and the tag is great compared to the wavelength of the carrier signal, or near-field technology, in which the operating distance is less than one wavelength of the carrier signal. In far-field applications, the RFID reader generates and transmits an RF signal via an antenna to all tags within range of the antenna. One or more of the tags that receive the RF signal responds to the reader using a backscattering technique in which the tags modulate and reflect the received RF signal. In near-field applications, the RFID reader and tag communicate via mutual inductance between corresponding reader and tag inductors.
In RFID systems that include passive tags and employ far-field technology, a passive tag's ability to generate power from a received RF signal directly correlates to the overall efficiency and effectiveness of an RFID system. In addition, such RFID tag power generation circuits need to be small and inexpensive. One such power generation circuit is a passive rectifier cell. As is known, a passive rectifier cell includes a plurality of diodes and capacitors where, in effect, the diodes steer energy of the RF signals into the capacitors to build up a voltage. The stored voltage is then used to power the tag. While a passive rectifier cell meets the design requirements fairly well, there is loss due to the threshold voltage of the diodes and capacitor leakage. In addition, the passive rectifier cell is not a voltage doubling circuit, thus, increasing the voltage after about three cell stages is limited.
Another known power generating circuit is a charge pump that includes a plurality of cells, where each cell includes two transistors and two capacitors. Each cell operates to build a charge in one capacitor through a corresponding transistor when the phase of the RF signal is between 0 and π and builds another charge in the other capacitor through its corresponding transistor when the phase of the RF signal is between π and 2π. The charges of the capacitors are summed to produce a cell voltage. The cells are cascoded to cumulate the cell voltages to produce the resulting output voltage.
While the charge pump power generating circuit enables a convenient CMOS implementation, its impedance limits the frequencies at which the circuit may be used and creates an impedance mismatch with most antenna structures. As such, for many RFID applications, a charge pump power generating circuit fails to provide an efficient power recovery unit.
Therefore, a need exists for a highly integrated, low-cost power generating circuit for a wide variety of RFID applications.